Event sourcing and CQRS are two key architecture concepts in the book that have less to do with the “micro” in “microservices,” and more to do with software architecture in general. While they are often mentioned together, they are separate concepts, and solve slightly different problems.
Event sourcing fits in the general category of an append only model, which is a way of persisting the data in your application. Instead of storing the current state, it stores history that led up to the present. It stores these using events, which are immutable, and represent business intent. Events record a thing that has happened.
A related concept in domain driven design is an aggregate, which rebuilds state by querying the event history.
CQRS stands for Command Query Responsibility Segregation. This relates to an OO principle, command query separation, which classifies methods into “commands,” which mutate the data (but does not return data), and “queries,” which return data only. In CQRS, we extend this principle to the design of a subdomain, and separate the responsibility of writing the data from reading the data. The two sides often communicate using events.
It solves a “stale data” problem in a collaborative domain. A collaborative domain is a business domain where multiple users are working together on a set of data, and expect that data to be coherent. The data is only as stale as the lag between the command side and the query side. However, by separating these sides and putting a visible part of the architecture between them, we have raised awareness that there will always be stale data in a collaborative system.
Another advantage to CQRS is that the read side and the command side can be scaled independently, so for example, if the number of queries is vastly greater than the commands, you can add more nodes that handle the queries.
For more on these concepts, see the CQRS Journey page on Microsoft, specifically the Exploring CQRS and Event Sourcing e-book. I’m also drawing from Patterns for Building Distributed Systems for The Enterprise, by Michael Perry on Pluralsight.